Drosophila germ band extension: a two-state reshaping mechanism

germ-band extension (GBE), a classic example of convergent-extension that extends the ventral germ band (GB) to the dorsal embryo surface. Using experimentally constrained biophysical modeling we find fluctuating internal cell-cell junctional myosin stresses excite cell intercalations that weakly perturb the anterior GB tissue into a fluidized state. Planar-polarized intercalation bias drives weak convergent-extension of the fluid. In the posterior GB, by contrast, external stress from adjacent tissue induces crystal-like ordering of the cells. External stress-induced cell intercalations mediate crystal defect annealing and plastic flow of the solid that wraps the GB around the narrow embryo posterior. Thus, the inhomogeneous reshaping challenge of GBE is dealt with by a two-state remodeling strategy in which a slowly remodeled anterior fluid coexists with a posterior solid undergoing fast plastic flow. Similar strategies find use in other contexts such as wound healing, where actomyosin boundary stresses drive crystal-like order that imposes shape regularity on the inner wound boundary for successful wound closure.